lacrima97
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Activity Series
I have been wondering for a while why would
2 Al+3 ZnCl2--->3 Zn+2 AlCl3
and Co+2 NaCl=no reaction.
I looked through my chem textbook and found a list of something called the activity series of metals. I am guessing there is a similar list for ions
such as NO3, NH4, SO4, blah blah.
Can anyone tell me why such a reaction as
2KI+Pb(NO3)2-->KNO3+PbI2
would occur? I don't under stand why K would take the NO3 and leave Pb with the I2. But i am guessing it has something to do with the activity
series.
Thanks.
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12AX7
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Nope- you need to realize the difference between ions, which are already oxidized, and reduction/oxidation reactions, which involve atoms or ions
changing oxidation state.
A reaction will only occur if it can. If you don't have enough voltage, you can't oxidize or reduce something. It takes a hell of a lot of voltage
to reduce sodium, and simply cannot be done at all, at any pH, in water, because for example if sodium did form, it will immediately react with nearby
H2O molecules, reducing it to H2 and taking the oxygen for itself. Water isn't a very good oxidizing agent, but when you have a strong reducing
agent, it's going to do something.
In the metathesis reaction 2KI + Pb(NO3)2 > 2KNO3 + PbI2, the lead starts in the +2 state (holding two NO3 ions, which hold a -1 charge each),
potassium in +1 and iodide in -1, and each of them end up in the same state, just rearranged. This reaction (and others) proceed because it's more
stable (somewhat lower energy state) with a crystalline (ordered) product. Depending on temperature and solvent, of course. (Some solvents are
better at picking apart crystals than others...water is pretty damn good though...)
Tim
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lacrima97
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Ohh, ok, then is there somewhere you learned this, or a chart, or something that can be looked at to see if a reaction will take place or not?
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chemoleo
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I always had the impression that the stronger acid anion (here NO3-) would go for the stronger base cation, which is K+ in this case.
For instance, this is why CuSO4 + Ca(Ac)2 --> CaSO4 + Cu(Ac)2 works.
This usually holds from what I have seen, except in cases where the product is extremely insoluble.
For instance, Pb(NO3)2 or Pb(Ac)2 + NaN3 --> Pb(N3)2 + NaNO3/NaAc works because lead azide is extremely insoluble (where HNO3/HAc is a MUCH
stronger acid than HN3). REmember, everything is at equilibrium, even if the energy differences are huge. Only a tiny fraction existing as Pb(N3)2 is
in solution, and in equilibrium. As it precipitates/is insoluble, it is taken OUT of the equilibrium, and more precikpitates, until the seemingly
unfavourable reaction has yet taken place. If lead azide was soluble, this reaction would not occur.
Thus I suppose the principle of metathesis is that the stronger acid will go for the stronger base. Strong defined by the degree of dissociation of HA
into H+ and A-, which is governed by the pKa, and a strong base defined by the dissociation of M(n)OH -> M(n)+ + OH-, whose equilibrium is defined
by pKb.
Never Stop to Begin, and Never Begin to Stop...
Tolerance is good. But not with the intolerant! (Wilhelm Busch)
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guy
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Where does Carbon lie in the activity series? It has the power to reduce metal oxides to metals (such as SnO2 and PbO) and H2O to H2 and CO. And it
can form carbides with some of them (why is that?).
[Edited on 1/20/2006 by guy]
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12AX7
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Carbon forms a lot of bonds in a lot of ways so I suspect it's too hard to pin down... might find voltages of a few specific reactions though.
Carbides, at least methanic (is that a word?) forms, such as SiC, TiC, WC, Al4C3, etc., are metal salt analogues of methane, crystallized. Of course
the crystal structure doesn't reflect that (methane is covalent and I would suspect solid (cryogenic!) methane has a crystal structure reflecting
individual molecules bonded by Van der Waals or whatever forces, not a crystallized soup of H+ and C(4-) ions), such materials as WC, HfC, TiC etc.
being cubic like salt (NaCl). There are three types of carbide structure, IIRC. Of course the most exciting carbide is the acetylide form, where
carbon bonds to itself as C2(2-) units, i.e., triple-bonded carbon. Uh, anyways, such inorganic reactions would be governed by electronegativity.
Tim
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guy
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Well Im interested in the carbon reduction of metal oxides to metals and metals to acetylides. For the acetylide formation in CaO + C --> CaC2 +
CO, what is going on with the carbon? I can see it reducing the Ba++ to barium then combining with the O. But then how does the C-C (in the
acetylide) triple bond form? Does the barium reduce the carbon? If so, then how can carbon reduce the Ba++ in the first place?
[Edited on 1/21/2006 by guy]
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12AX7
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Because it wants to.  (Ah, the reason behind damned everything in physics and
thus chemistry...)
I have no idea why Ca forms acetylide specifically.
Carbon is a strong reducing agent, I think because CO is a stable, gaseous molecule. it's capable of reducing half the periodic table by the
equilibrium C(s) + RO(s/l) > CO(g) + R(s/l). However, some elements form carbides either so stubbornly or carbon has so strong an effect that
smelting with carbon is useless (iron doesn't form carbides in equilibrium, but a mere 0.5% C can cause quenched iron to become very hard!). This
constitutes most of the left end of the transition elements.
As for the alkali earth metals, these have low boiling points so tend to form vapors. Zinc, and congeners such as cadmium are smelted in this way,
too, but they aren't as reactive so happily condense into liquid state. The AEs, however, are in fact reactive enough to reduce CO. The only way you
can carbothermically smelt magnesium is to seperate the vapors. One way this is done is, first the vapors are generated by C + MgO by electric arc,
then they are shot to a water-cooled plate and quenched rapidly, leaving a magnesium regulus (with some carbon impurity, IIRC) and CO gas. The vapors
are equilibrium Mg + CO <--> MgO + C, so inevitably, some carbon will fall out, while some Mg will condense. The other way, is a method
proposed for lunar processing. You use a large solar array to heat sand to a scary temperature, and I think C is needed, and the Mg + CO vapors waft
out, with CO floating and Mg sinking (I don't know how they control convection mixing...). Condense the vapors, deoxygen the CO, and you've got a
continuous process. Oh- in the vacuum of space, you have a lower reaction temperature, Mg being more volatile.
Tim
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Magpie
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I think stronger (more ionized) species tend to react to form weaker (less ionized) species. An example would be strong base NaOH reacts with strong
acid HCl to form NaCl and weak acid (or weak base) HOH even though this product is in great abundance in an aqueous solution.
Then take the reaction H2SO4 + CaF2 - > CaSO4 + 2HF
HF is a weak acid, but CaF2 is more insoluble than CaSO4, so what happens? Does the reaction proceed? It is usually done with some heating which
drives off the HF. I think this is the key to the reaction proceeding. But I'm not an expert in this area.
Another factor which can decide reaction progress is the size and sign of the net change in Gibbs free energy. If it is positive the equilibrium
conditions will be very unfavorable.
There is also the issue of kinetics. Some reactions do not proceed fast enough to be of practical value.
[Edited on 21-1-2006 by Magpie]
The single most important condition for a successful synthesis is good mixing - Nicodem
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neutrino
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The reaction is usually driven forward by the volatile HF evaporating. Remember also that the salts of weak acids (e.g. fluorides, sulfates) are
significantly more soluble in strong acids. The reaction isn't as solid-state as one may think.
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